An induction furnace employs electromagnetic energy to induce electrical currents within a charge of metal or metal alloy. The electrical resistance of the metal produces heat as a natural consequence of the induced currents flowing in the metal. The combination of applied electrical power and frequency can be chosen to induce sufficient heat within the metal to cause it to melt, providing a molten liquid which can be poured into molds or otherwise used to produce a wide variety of metal products.
The basic elements of an induction furnace include an electromagnetic induction coil, a vessel having a lining of refractory material, and a support structure for the coil and vessel.
The coil must comprise an electrical conductor of sufficient size and current capacity to produce the magnitude of electromagnetic flux necessary to induce large currents in the metal charge. Often, the coil must be cooled to prevent it from being damaged by overheating. A common cooling technique is to fabricate the coil from hollow tubing through which water is passed to carry away the heat. The water-cooled induction coil is usually either very close to or in contact with the exterior of the refractory material containing the metal charge.
Within the furnace is a vessel which is lined with refractory material to withstand the extreme temperatures associated with molten metals. Refractory material can withstand extremely high temperatures without deformation. The refractory lining nearest the molten metal is generally a packed granular material. The inner surface of the packed granular lining is the crucible. The crucible is the container for the melt. In smaller furnaces, the crucible may be a preformed refractory container around which the packed granular lining is rammed into place. In larger furnaces the packed granular lining material packed into the furnace is the crucible. It becomes fused on the interior surface after contact with molten metal the first time the furnace is used. A solid refractory material may also be used to line and insulate the bottom of the furnace vessel.
In operation, the interior surface of the refractory lining that contacts the molten metal becomes sintered and brittle because of the extreme temperatures to which it is exposed. As the furnace is used repeatedly, the refractory expands and contracts in response to the heating and cooling cycles. Cracks form in the refractory, permitting small amounts of molten metal to migrate into the granular material.
The crucible refractory and lining typically is replaced after a preselected number of melting cycles to prevent failures of the furnace. However, there is always the danger that an unexpected failure can occur. This danger is particularly acute when the furnace is used to superheat the molten metal. Superheated metal can accelerate the breakdown of the refractory lining of the furnace. If a failure occurred in the wall of the crucible lining such that superheated molten metal within the crucible were able to flow through the refractory and reach the water-cooled induction coil, a catastrophic water-molten metal explosion could result.
There is a need to provide for a controllable failure mode in induction furnaces of this type so that, if a failure does occur in the furnace lining, it will occur in such a manner that runout of molten metal will occur in a preselected location away from the induction coil, and away from operating personnel and equipment. The present invention fills that need.